This invention deals with the cyclic non-invasive introduction of soluble and insoluble inert gases into the airway of a human patient for purposes of obtaining continuous measurements of such pulmonary parameters as pulmonary blood flow, ventilation-perfusion distribution and functional residual capacity.
In existing theory and practice, gas exchange of soluble inert gas to determine pulmonary blood flow (Q.sub.c) has been used. However, the earlier techniques contained certain inherent problems as to the manner of injection and sampling of the soluble gases. For example, with the plethysmographic and rebreathing methods, a willing and cooperative patient is required for achieving best results. This limits the usefulness for testing of neonates, young children, and patients who are unconscious or unwilling to participate. The rebreathing method has the additional disadvantage of requiring an airtight system. In addition, it cannot be used continuously to monitor a patient's condition due to recirculation of the soluble inert gas. Another major problem with the breath holding and rebreathing techniques is that they disturb the parameters being measured. A third major problem is that both the rebreathing and breath-holding techniques are based on a uniform or homogeneous model of the lung. This representation is not usually valid for patients with significant pulmonary disease.
A method for measuring the ventilation-perfusion distribution requires continuous intravenous injections of soluble gases of differing solubilities as well as periodic sampling from the central venous system. Blood sampling requires the use of a catheter which passes into the pulmonary system through the heart. Not only is this method highly invasive but is is noncontinuous as well. Therefore, the range of practical usefulness of the method is very limited.
It would be of great value to obtain such parameters as pulmonary blood flow, the ventilation-perfusion distribution, and functional residual capacity (FRC) by non-invasive means which require minimal or no patient cooperation. Among those with pulmonary diseases, continuous monitoring of such parameters would be an important and helpful means of diagnosis and treatment. For example: reduced pulmonary blood flow could indicate PFC (persistant fetal circulation) in a neonate, a decrease in FRC (functional residual capacity) might indicate collapsed alveoli present in RDS (Respiratory Distress Syndrome) patients, and significant ventilation in high V/Q regions could indicate improper mechanical ventilation or the presence of pulmonary emboli.
The importance of these parameters in the diagnosis and treatment of ill patients, many of whom could be uncooperative is apparent. There is also a use and reason for knowing and obtaining such parameters in normals. The growing number of health spas and the importance placed on exercising creates a use for testing to determine such parameters as pulmonary blood flow (Q.sub.c), oxygen uptake, and carbon dioxide production. Such knowledge could lead to better exercise programs and would lead to a better characterization of a healthy physique and its parameters. Overall, there is a need to develop and employ a non-invasive method of continuously monitoring and measuring the above mentioned pulmonary parameters.